1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly to a semiconductor device that is manufactured by processes including radiating light to the semiconductor device and a semiconductor device that has a thin film element such as a resistor with respect to which trimming is performed using a laser beam.
2. Description of the Related Art
There is a known laser trimming method in which a resistor material such as CrSi is deposited on a semiconductor substrate through an insulating material or the like, and patterning is performed thereon. Subsequently, a prescribed pattern, and then a laser beam is radiated onto the resulting thin film resistor to thereby fuse a part thereof and set its resistance value to a desired value.
In the above laser trimming, the laser beam radiated onto the thin film resistor transmits through the thin film resistor, and is reflected by an interface between an underlying oxide film layer and the semiconductor substrate that interferes with the incident laser beam. Due to this interference, stable trimming of the thin film resistor is impossible to perform.
U.S. Pat. Nos. 4,594,265 and 4,708,747 disclose techniques which solve this problem. Each of these techniques concerns a semiconductor device wherein respective semiconductor layers are electrically isolated by providing an isolating insulative film within a semiconductor substrate. Further, an insulating layer is formed on the surface of the semiconductor substrate and a thin film resistor is formed thereon. In the former technique, indentures or V-grooves are formed by etching or the like in the surface of the semiconductor layer located at the interface between the isolating insulative film within the semiconductor substrate and the semiconductor layer to thereby prevent the laser beam reflection from the interface between the isolating insulating film within the semiconductor substrate and the semiconductor layer. In the latter technique, the laser beam reflection is prevented by controlling the thickness of the isolating insulative film to a value that permits this isolating insulative film to become "transparent" with respect to the laser beam.
However, in the techniques disclosed in the above Patents, no consideration is given to the transmission light of the laser beam that is reflected by the interface between the insulating layer right under the thin film resistor that has been formed on the semiconductor substrate and the surface of the semiconductor substrate.
The Journal of Nippondenso Technical Disclosure No. 87-023 (published Nov. 15, 1992), as shown in FIG. 16 discloses a countermeasure. The countermeasure technique use a thin film resistor 3 formed on an Si substrate 1 with an insulating layer 2 interposed therebetween. Further, at an interface between the Si substrate 1 and the insulating layer 2 located under the thin film resistor 3, there is provided a level difference portion A having a height corresponding to 1/4 of the laser beam wavelength to thereby cause the laser beams reflected by upper portions and lower portions of concavities and convexities in the level difference portion A to cancel one another. As a result, the incident laser beam and reflected laser beam are prevented from interfering with each other in the vicinity of the thin film resistor. This enables laser trimming to be performed with a high degree of precision.
However, in this technique, precise control of the height of the top portion and the bottom portion defined by the concavities and convexities of the level difference portion is required to cancel the interference due to the reflected laser beam.
On the other hand, the laser trimming is performed by using an alignment mark. A second Al (second Al wire layer), which is maintained on a scrub line between chips, has been used as the alignment mark.
However, since it has recently been desire to reduce the scrub line, level difference portions are generated at both sides of the scrub line. In this situation, it has been found that the alignment may be shifted, because the process becomes difficult. A surface roughness is generated due to etching remains or over-etching, and a reflection intensity of the laser beam radiated to the alignment recognition is influenced.
A reason for the above-mentioned alignment shift will be explained in detail. FIG. 21A shows a sectional view of the element in which an alignment mark 52 is formed on a scrub line of an SOI substrate 51. As shown in FIG. 21B, when a laser beam is radiated to around a second Al as the alignment mark 52, a reflection intensity is high at the second Al, while the reflection intensity is low at surrounding area of the second Al. That is, the second Al reflects the laser beam largely because the second Al does not transmit the laser beam, while the surrounding area does not provide a large amount of reflection. Therefore, the alignment mark 52 can be recognized precisely based on a difference of the reflection intensities when the laser beam is radiated thereto.
However, when the surface roughness is generated, since the reflection intensity may vary within a range of 0-80% (see the arrows in the figure), the alignment may not be precisely recognized.
Here, the alignment mark for trimming may be disposed in a ship instead of on the scrub line, where the recognition of the alignment mark is difficult to improve the recognizablity of the alignment mark and to secure a disposition space of the alignment mark for trimming.
Furthermore, a stepper method has been recently utilized to secure an efficient chip number. In the stepper method, data such as a photography alignment mark for a photography or TEG pattern is disposed on the scrub line. Therefore, it becomes difficult to secure the disposition space of the alignment mark for trimming on the scrub line. Hence, it would be effective to dispose of the need for the alignment mark in the chip.
However, in such a case, as shown in FIG. 22A, which is a sectional view of the element that the alignment mark is disposed in the chip, an internal structure of the chip may become complicated. That is, the element includes insulating films 61, 62 formed by, for example, as a TEOS film or a P--SiN as an interlayer insula ting films, an insulating film 63, a BPSG film 64, Si film 65 that constitutes an Si substrate, a buried SiO.sub.2 66 which are laminated on the Si substrate 67 below the second Al 60. Therefore, as shown in FIG. 22B, the reflection intensity around the surrounding area of the second Al 60 may vary due to a thickness variation of the films 61-67.
Therefore, it may not be possible to improve t he recognizablity of the alignment mark or may not be possible to secure the disposition space of the alignment mark for trimming.